Design Challenges for Wax in a Fast-Track Deepwater Project

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 19160, "Design Challenges Due to Wax on a Fast- Track Deepwater Project," by Amir Alwazzan, SPE, and Mark Utgard, SPE, AMEC Paragon, and Dalmo Barros, Petrobras America, prepared for the 2008 Offshore Technology Conference, Houston, 5-8 May. The paper has not been peer reviewed. Flow-assurance threats, such as hydrates, waxes, and asphaltenes, can be identified and controlled if oil and gas production and transportation systems are designed accordingly and proper operating procedures are implemented. Therefore, prevention of wax deposition is key for good subsea deepwater-system design. Wax deposition can form a blockage and impede flow, causing weeks of lost production and operating difficulties. Introduction Flow assurance in subsea systems is an issue in the design of deepwater-field developments. Flow-assurance efforts focus on preventing solid deposits from blocking or restricting the rate of flow from the well. The principal solids of concern are wax and hydrates. For a given reservoir fluid, these solids precipitate at certain combinations of pressure and temperature. Precipitated solids often are carried downstream, slurried in the fluid, but precipitated solids also can deposit on the walls of the production equipment, which ultimately causes high pressure drops, plugging, and flow stoppage. Control of this deposition by prevention and/or mitigation is the essence of flow assurance. Wax in hydrocarbons is primarily paraffin, which is a white, odorless, tasteless, chemically inert compound composed of saturated hydrocarbons. The linear paraffins are easily measured by high-temperature gas chromatograph (HTGC). Two critical issues require special attention: gel formation (precipitation) and deposition. A hydrocarbon gel forms when wax precipitates from the oil and forms a 3D structure spanning the pipe. This condition does not occur while the fluid is flowing because the intermolecular structure is destroyed by shear forces as soon it is able to form. However, when the fluid stops flowing, wax particles will interact and join together, potentially forming a network resulting in a gel structure if enough wax comes out of solution. In a pipe, wax deposition results in flow restrictions. Slow buildup of wax layers in pipelines and flowlines is caused by the solidification of the paraffinic fractions on the cold inner surface of the pipe wall. Wax deposition is expected during steady-state operation in the flowline where the production-stream temperature is below the wax-appearance temperature (WAT). The deposition rate depends mainly on the fluid characteristics, flowing and pipe-wall temperatures, heat flux through the pipeline, and the shear stress at the wall.